- 學位論文
基於離散傅立葉轉換的正交分頻多工頻譜緊密訓練波形
Spectrally Compact DFT-Based OFDM Training Waveforms
摘要
現有的等振幅序列旨在正交分頻多工系統中建構頻譜緊密的訓練波形以實現準確的通道估測、穩定的初始時間和頻率同步並提供緊密的頻譜。而目前已設計過的等振幅序列是基於具有無限大功率頻譜的類比矩形脈沖多載波波形設計的。所以直接使用離散傅立葉轉換將目前設計的等振幅序列所產生的類比波形轉換為數位波形是無法契合實際使用基於離散傅立葉轉換的發射端。因此，希望在基於離散傅立葉轉換的正交分頻多工系統中調整這些現有的等振幅序列，使這些序列適用於此系統。在本論文中，針對使用過取樣的離散傅立葉轉換正交分頻多工系統，發展出對現有等振幅序列的條件以得到基於離散傅立葉轉換訓練波形提供的功率頻譜旁波帶包絡邊界漸近衰減為 𝑓−2𝐽−2。結果證明，透過使用適當的對角相位旋轉矩陣轉換適用於以類比多載波波形而設計的等振幅序列可以獲得適用基於離散傅立葉轉換波形的序列。
並列摘要
Recently, frequency-domain constant-amplitude (CA) sequences have been designed for an extensive set of sequence lengths to construct spectrally compact training waveforms in orthogonal frequency-division modulation (OFDM) systems, which can provide small power spectral sidelobes and achieve accurate channel estimation, robust initial time and frequency synchronization. However, these existing CA sequences are designed in the rectangularlypulsed analog multicarrier waveform representation which are difficult to realize for a large number subcarriers. Moreover, the rectangularly-pulsed analog multicarrier waveform is extended over a power spectrum of infinite bandwidth, these spectrally compact training waveforms can not be exactly synthesized by a practical transmitter by which the target analog multicarrier waveform is digitally synthesized with the use of discrete Fourier transform (DFT). Consequently, these existing CA sequences are subject to modification. The synthesized DFT-based training OFDM waveforms can provide small power spectral sidelobes and achieve accurate channel estimation, robust initial time and frequency synchronization. In this thesis, general constraints on the existing CA sequences are developed for DFT-based OFDM with oversampling to ensure the desirable spectral property that the resultant DFTbased training waveforms provide power spectral-sidelobe envelope bounds decaying asymptotically as 𝑓−2𝐽−2. It is shown that the sequences suitable to DFT based waveforms can be obtained by transforming the previously designed CA sequences applicable to the corresponding analog multicarrier waveforms with a suitable diagonal phase-rotation matrix. At the DFT-based OFDM receiver, the same level of accurate channel estimation, as well as robust initial time and frequency synchronization can also be sustained since these transformed CA sequences still possess the frequency-domain constant-amplitude characteristic.